The present invention relates to a process and device for accelerating the destruction of at least two vortices in the wake of a moving body.
The present invention is applied more particularly, but not exclusively, to an aircraft, especially a transport aircraft.
In this case, the two (or more) vortices to which the present invention is applied are generally created when the aircraft is taking off or landing, and at least one flap, particularly a wing flap, is deployed on each wing of the aircraft. These two vortices are contra-rotating and one of them is created in each of the two wakes of the said wings of the aircraft, each of the said vortices being formed behind the corresponding wing by the merging of at least two co-rotating eddies, one of which is created by the wing tip and the other by the deployed flap. These co-rotating eddies are caused by the pressure gradients existing at the ends of the said aerofoils (wing tip, flap) between the lower and upper surfaces of these aerofoils. The pressure gradient forces the fluid to flow around the end of each aerofoil, creating a rotary movement of the fluid which generates the eddy. In cruising flight, there are also contra-rotating vortices which are generated by the wing tips only, but a device for accelerating their destruction would tend to impose excessive drag.
To avoid any confusion, the following terms will be used in relation to the present invention:
xe2x80x9ceddiesxe2x80x9d, denoting the eddy phenomena existing before merging, which will merge with each other; and
xe2x80x9cvorticesxe2x80x9d, denoting eddy phenomena caused by the merging of at least two of these eddies.
The two contra-rotating vortices which are created in the wake of the aircraft can be very energetic and can produce a velocity field which is highly destabilizing for a following aircraft (this velocity field is commonly called the xe2x80x9cwake turbulencexe2x80x9d), and, in particular, may cause a powerful rolling moment and a powerful downward motion of the air. Moreover, they have a significant duration (several minutes), and therefore make it necessary to maintain large separation intervals between aircraft in the vicinity of airports.
A fixed separation grid, based on the weights of the aircraft, is currently used to establish the separation intervals between two aircraft at take-off and landing. Since air traffic is constantly increasing, the frequency of take-offs and landings is thus limited at many airports by excessively large separation intervals.
The object of the present invention is, in particular, to accelerate the destruction of this pair of contra-rotating vortices created in the wake of an aircraft at take-off and landing.
There are various types of known device designed to act on different wake eddies of aircraft.
In particular, the document U.S. Pat. No. 5,492,289 discloses a method for accelerating the destruction of an eddy created in the wake of an aircraft by a wing tip or by a flap carried by a wing. This known document proposes that the trailing edge of the wing or flap be modified in such a way as to modify the distribution of lift along the corresponding wing. Such a modification of the lift causes a faster increase in the diameter of the (wing-tip or flap) eddy and thus accelerates its destruction. However, the efficacy of this method is questionable, particularly as regards its effect on the actual acceleration of the destruction. Moreover, this known solution requires a modification of the wing geometry, which entails practical problems of implementation.
The document U.S. Pat. No. 6,042,059 discloses another system and method for destroying wake eddies of an aircraft more rapidly. This known method entails the use of small aerodynamic surfaces to generate a parasitic eddy designed to initiate the process of destruction of the wake eddies. These small aerodynamic surfaces are streamlined and arranged on the lifting surfaces of the aircraft. However, they have the drawback of increasing the drag.
Additionally, the document WO-99/00297 discloses an active system for destroying the wake eddies of an aircraft. This known system is based on the active excitation of the multiple instabilities of eddies by the movable surfaces of the aircraft wings, but without changing the internal structure of these eddies, and without exciting an internal instability of the cores of these eddies. This known active system is highly complex, since it requires the use of a computer and synchronized commands for moving the ailerons and spoilers in a controlled way during the flight. Consequently, there may be effects on the manoeuvrability of the aircraft, the control of its lift, the stress on its structure and the comfort of passengers. Moreover, the efficacy of this known system has not been proved.
The object of the present invention is to overcome the aforesaid drawbacks. It relates to a process for accelerating the destruction of at least two contra-rotating vortices which are generated in the wake of a moving body, particularly in the wake of an aircraft when the flaps are deployed on the said aircraft, in other words at the take-off and landing stages.
For this purpose, the said process for accelerating the destruction of a pair of contra-rotating vortices which are created in the wake of a moving body having at least two wings, where each wing has at least two lifting surfaces, and which are separated from each other by an inter-vortex distance, each of the said contra-rotating vortices being formed at the rear of the corresponding wing by the merging of at least two co-rotating eddies which are created by the arrangement of the said lifting surfaces of the wing, each of the said co-rotating eddies having a plurality of core instability modes, is characterized in that a periodic perturbation of the flow is generated on each of the said wings, in the vicinity of the area of creation of at least one of the said co-rotating eddies associated with the wing, and in that each of the said periodic perturbations has a wavelength capable of exciting at least one of the said instability modes of the corresponding eddy in such a way as to increase the core of the contra-rotating vortex which is created by the unstable merging of this eddy with the other eddy, so that the diameter of the said core becomes greater than a predetermined proportion of the said inter-vortex distance.
Thus, owing to the periodic perturbations generated according to the present invention, the ratios between the core diameters of the two vortices on the one hand, and the inter-vortex distance on the other hand, become greater than a predetermined critical value, above which the two vortices begin to interact strongly and are rapidly destroyed. This is because, as mentioned above, when this critical value is reached there is an exchange of fluid between the two contra-rotating vortices, with the creation of smaller secondary eddies, which are perpendicular to the axes of the said vortices. This exchange of fluid causes a rapid and marked decrease in the circulation in each vortex, and consequently the disintegration or destruction of the said pair of contra-rotating vortices.
The process according to the present invention therefore makes it possible to reduce the duration of the said contra-rotating vortices and thus overcome the aforesaid drawbacks.
The present invention is more particularly, but not exclusively, applicable to an aircraft. In this case, the two lifting surfaces of a wing, each of which generates one of the said co-rotating eddies, are generally the surface of the wing tip on the one hand and a deployed flap on the other hand. However, the present invention may also be applied to other moving bodies, particularly a submarine, which creates vortices in the wake of its fins.
The said instability mode of the core to be excited is advantageously determined from the size of the cores of the eddies and the ratios between the sizes of the cores and the distance between the eddies. The said instability mode is preferably determined empirically. Generally, the wavelength of the instability mode is essentially equal to the mean diameter of the corresponding eddy core.
Additionally, and advantageously, the wavelength of a perturbation to be generated is:
of the order of a divisor of the most unstable wavelength of the instability mode which it is to excite, permitting a reduction of the size of the means used to generate the said perturbation; and/or
located within an instability range of each of the co-rotating eddies of the corresponding wing.
The present invention also relates to a device for accelerating the destruction of a pair of vortices such as those described above.
According to the invention, the said device is characterized in that it comprises, on each of the said wings, at least one perturbation means which is positioned in the vicinity of the area of creation of one of the said co-rotating eddies associated with the wing, and in that each of the said perturbation means can generate a periodic perturbation of the flow, which has a wavelength capable of exciting at least one of the said instability modes of the corresponding eddies in such a way as to increase the core of the contra-rotating vortex which is created by the merging of this eddy with the other co-rotating eddy in such a way that the diameter of the said core becomes greater than a predetermined proportion of the said inter-vortex distance.
Advantageously, at least one of the said perturbation means comprises:
in a first embodiment, an unstreamlined element, for example a cylinder, whose apparent diameter which is transverse with respect to the flow depends on the wavelength of the periodic perturbation to be generated; and
in a second embodiment, a means for producing a jet of fluid emitted transversely with respect to the said flow. If the said jet of fluid is emitted orthogonally to the flow, its velocity must be at least equal to that of the moving body and its diameter must be of the same order of magnitude as the apparent diameter of an unstreamlined element which could be used in its place.
The device according to the invention is therefore easily constructed and inexpensive. Moreover, it is passive and very robust.
Additionally, at least one of the said perturbation means is advantageously retractable. For this purpose, the said perturbation means is preferably retractable into the wing or into fairings fitted on the wing (the strut end fairing, for example) or into the flap. This avoids increasing the drag and thus avoids degrading the performance of the moving body, particularly in the absence of contra-rotating vortices, especially in cruising flight in the case of an aircraft.